1. Field of the Invention
The present invention relates to an optical encoder which detects a rotation position of a rotatable member based on the reflected light which is obtained by emitting light toward a rotary type code plate.
2. Description of the Related Art
In the field of industrial machinery, various encoders are used as sensors for detecting the positions of electric motors or equipment which are driven by electric motors. In particular, optical encoders are broadly employed for the purpose of detecting the rotation positions of drive shafts of rotary motors. In general, optical encoders may be classified into transmissive types which detect rotation positions of rotatable members based on transmitted light which is obtained by emitting light toward rotary type code plates, or reflective types which detect rotation positions of rotatable members based on reflected light which is obtained by emitting light toward rotary type code plates. A transmissive type encoder has a structure in which with the light emitting diode and the light receiving diode are arranged along the rotation axe so as to interpose the code plate. On the other hand, a reflective type encoder has a structure in which the light emitting diodes and the light receiving diodes face the same major surface of the code plate. Therefore, a reflective type encoder has the advantage of being easily downsized in the direction of the rotation axis, compared with a transmissive type encoder.
In relation to this, JP-A-H11-287671 proposes a reflective type encoder which is provided with a code plate formed from a transparent plastic material, while JP-B-3730232 proposes a reflective type encoder which is provided with code plate which is formed from a polyether imide, polyether sulfone, polyphenyl sulfone, or other colored plastic material. If these plastic materials are used for a code plate, it is possible to easily and inexpensively manufacture a code plate with complex code patterns by means of injection molding or other plastic forming techniques.
More specifically, the code plate described in JP-A-H11-287671 is provided with a code pattern which is comprised of V-shaped groove parts and planar parts which are formed on a second major surface on the opposite side to the first major surface which faces the light emitting diode, and is configured to switch transmission/reflection of light by means of these groove parts and planar parts. In order to ensure the total reflection by the planar parts of the second major surface, the incident angle of the light with respect to the planar parts has to be equal to or more than a predetermined critical angle, but due to refraction at the first major surface, the incident angle of the light with respect to the flat parts becomes smaller than the incident angle of the light with respect to the first major surface. Therefore, if the code plate described in JP-A-H11-287671 is employed, the incident angle of the light with respect to the first major surface has to be sufficiently large, and this may cause a reduction in the degree of freedom of arrangement of light emitting diodes with respect to the code plate.
Further, the code plate described in JP-B-3730232 is provided with a code pattern comprised of projecting parts with triangular cross-sections and planar parts which are formed on a second major surface on the opposite side to the first major surface facing the light emitting diode, and is configured to switch total reflection/transmission of light by means of the projecting parts and planar parts. In the code plate described in JP-B-3730232, the projecting parts on the second major surface are configured to reflect the light vertically entering the first major surface perpendicularly to the first major surface. However, if the light receiving diode is arranged on the route of such reflected light, there is a risk that part of the light heading from the light emitting diode toward the first major surface is blocked by the light receiving diode.
A reflective type optical encoder which can improve a degree of freedom of arrangement of a light emitting diode with respect to a code plate is therefore being sought.